Signal transmission line and circuit board

ABSTRACT

A signal line and a circuit board that can be easily bent in a U shape and prevent unwanted emission include a line portion includes a plurality of laminated line portion sheets made of a flexible material. Signal lines extend within the line portion in an x-axis direction. Ground lines are provided within the line portion on a positive direction side in a z-axis direction with respect to the signal lines and have line widths equal to or smaller than the line widths of the signal lines. Ground lines are provided within the line portion on a negative direction side in the z-axis direction with respect to the signal lines. The signal lines overlap the ground lines when seen in a planar view from the z-axis direction.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a signal transmission line and acircuit board, and more specifically, relates to a signal transmissionline and a circuit board in which a high-frequency signal istransmitted.

2. Description of the Related Art

As existing signal transmission lines, for example, the microstrip lineand the strip line that are described in Japanese Unexamined PatentApplication Publication No. 2004-152963 are known. FIG. 8A is across-sectional structure diagram of the microstrip line 200 a describedin Japanese Unexamined Patent Application Publication No. 2004-152963,and FIG. 8B is a cross-sectional structure diagram of the strip line 200b described in Japanese Unexamined Patent Application Publication No.2004-152963. In FIGS. 8A and 8B, the up-down direction is defined as alamination direction.

The microstrip line 200 a shown in FIG. 8A is composed of a main body202, a signal line 204, and a ground conductor 206. The main body 202 iscomposed of, for example, a plurality of laminated insulating sheetsformed from a flexible material. The signal line 204 is a wiring inwhich a high-frequency signal is transmitted, and extends in thedirection perpendicular to the sheet surface of FIG. 8A. A groundpotential is applied to the ground conductor 206, and the groundconductor 206 is provided on the lower side of the signal line 204 inthe lamination direction. The signal line 204 and the ground conductor206 overlap each other when seen in a planar view from the laminationdirection.

The microstrip line 200 a described above is used, for example, forconnecting a signal generation source to a load circuit in a state whereimpedance matching is provided therebetween. Specifically, in themicrostrip line 200 a, a capacitance occurs between the signal line 204and the ground conductor 206, and the impedance of the signal line 204decreases. Thus, impedance matching is provided among the signalgeneration source, the signal line 204, and the load circuit.

However, in the microstrip line 200 a, unwanted emission occurs.Specifically, a high-frequency signal has a wavelength shorter than theelectrical length of the microstrip line 200 a. Thus, when ahigh-frequency signal is transmitted in the signal line 204, a pluralityof standing waves is present in the signal line 204. Due to the standingwaves, noise is emitted from the signal line 204. Of the noise, thenoise emitted to the lower side in the lamination direction is absorbedby the ground conductor 206. However, the noise emitted to the upperside in the lamination direction is emitted from the signal line 204 tothe outside of the main body 202.

Meanwhile, in the strip line 200 b shown in FIG. 8B, unwanted emissioncan be suppressed. Specifically, the strip line 200 b has a structure inwhich a ground conductor 208 is provided in the microstrip line 200 a.As shown in FIG. 8B, the ground conductor 208 is provided on the upperside of the signal line 204 in the lamination direction. The signal line204 and the ground conductor 208 overlap each other when seen in aplanar view from the lamination direction. In the strip line 200 bhaving the structure described above, noise emitted from the signal line204 is absorbed by the ground conductor 208. Thus, in the strip line 200b, unwanted emission can be suppressed.

However, the strip line 200 b has a problem that it is difficult to bendthe strip line 200 b in a U shape. Specifically, the strip line 200 b iscomposed of three conductor layers, namely, the signal line 204 and theground conductors 206 and 208. The signal line 204 and the groundconductors 206 and 208 are in general produced from a metal film such ascopper foil, and thus it is difficult to bend these elements as comparedto the insulating sheets constituting the main body 202. Therefore, thestrip line 200 b has a problem in that it is relatively difficult tobend the strip lines 200 b in a U shape.

As described above, in the existing microstrip line 200 a and strip line200 b, it is difficult to achieve both suppression of unwanted emissionand easy bending in a U shape.

SUMMARY OF THE INVENTION

Therefore, preferred embodiments of the present invention provide asignal transmission line and a circuit board that can easily be bent ina U shape and that can prevent unwanted emission.

A signal transmission line according to a preferred embodiment of thepresent invention includes a line portion including laminated insulatingsheets made of a flexible material; a signal line extending within theline portion; a first ground line provided within the line portion onone side in a lamination direction with respect to the signal line andhaving a line width equal to or smaller than a line width of the signalline; and a second ground line provided within the line portion onanother side in the lamination direction with respect to the signalline. The signal line overlaps the first ground line and the secondground line when seen in a planar view from the lamination direction.

A circuit board according to another preferred embodiment of the presentinvention includes a main body that includes a first board portion andsecond board portion on which electronic components are mounted, and aline portion connecting the first board portion to the second boardportion and that includes a plurality of laminated insulating sheetsmade of a flexible material; a signal line provided within the lineportion and extending between the first board portion and the secondboard portion; a first ground line provided within the line portion onone side in a lamination direction with respect to the signal line andhaving a line width equal to or smaller than a line width of the signalline; and a second ground line provided within the line portion onanother side in the lamination direction with respect to the signalline. The signal line overlaps the first ground line and the secondground line when seen in a planar view from the lamination direction.The first board portion and the second board portion have structuresthat are less likely to deform than the line portion.

According to various preferred embodiments of the present invention,easy bending in a U shape can be performed and unwanted emission can beprevented.

The above and other elements, features, steps, characteristics andadvantages of the present invention will become more apparent from thefollowing detailed description of the preferred embodiments withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an external perspective view of a circuit board according to apreferred embodiment of the present invention.

FIG. 2 is an exploded perspective view of the circuit board in FIG. 1.

FIGS. 3A and 3B are perspective views of a flexible sheet of a circuitboard in a manufacturing process.

FIG. 4 is a cross-sectional structure diagram along A-A in FIG. 1.

FIG. 5A is a cross-sectional structure diagram of the circuit board thatis bent in a U shape, and FIG. 5B is an enlarged view at B in FIG. 5A.

FIG. 6 is a cross-sectional structure diagram of a line portionaccording to a first modified example of a preferred embodiment of thepresent invention.

FIG. 7 is a cross-sectional structure diagram of a line portionaccording to a second modified example of a preferred embodiment of thepresent invention.

FIG. 8A is a cross-sectional structure diagram of the microstrip linedescribed in Japanese Unexamined Patent Application Publication No.2004-152963, and FIG. 8B is a cross-sectional structure diagram of thestrip line described in Japanese Unexamined Patent ApplicationPublication No. 2004-152963.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, a signal transmission line and a circuit board according topreferred embodiments of the present invention will be described withreference to the drawings.

Hereinafter, the configuration of a circuit board according to variouspreferred embodiments of the present invention will be described withreference to the drawings. FIG. 1 is an external perspective view of thecircuit board 10 according to a preferred embodiment of the presentinvention. FIG. 2 is an exploded perspective view of the circuit board10 in FIG. 1. FIGS. 3A and 3B are perspective views of a flexible sheet26 a of the circuit board 10 in a manufacturing process. FIG. 3A shows aback surface of the flexible sheet 26 a, and FIG. 3B shows a frontsurface of the flexible sheet 26 a in a state where resist films 20 and24 are not formed thereon. FIG. 4 is a cross-sectional structure diagramalong A-A in FIG. 1. In FIGS. 1 to 4, a lamination direction of thecircuit board 10 is defined as a z-axis direction, and a longitudinaldirection of a line portion 16 of the circuit board 10 is defined as anx-axis direction. The direction perpendicular to the x-axis directionand the z-axis direction is defined as a y-axis direction. It should benoted that front surfaces of the circuit board 10 and a flexible sheet26 refer to surfaces located on a positive direction side in the z-axisdirection, and back surfaces of the circuit board 10 and the flexiblesheet 26 refer to surfaces located on a negative direction side in thez-axis direction.

As shown in FIG. 1, the circuit board 10 includes board portions 12 and14 and the line portion (signal transmission line) 16. The board portion12 preferably has a rectangular or substantially rectangular shape andhas, in the front surface thereof, a mounting surface on which aplurality of chip elements (electronic components) 50 and an integratedcircuit (electronic component) 52 are mounted. The board portionpreferably has a rectangular or substantially rectangular shape smallerthan that of the board portion 12 and has, in the front surface thereof,a mounting surface on which a connector (electronic component) 54 ismounted. The line portion 16 connects the board portion 12 to the boardportion 14. As shown in FIG. 2, the board portions 12 and 14 and theline portion 16 include laminated flexible sheets 26 (26 a to 26 d) madeof a plurality (four in FIG. 2) of flexible materials (e.g., athermoplastic resin such as a liquid crystal polymer), for example.

First, the board portion 12 will be described. As shown in FIG. 2, theboard portion 12 preferably includes laminated board portion sheets 27 ato 27 d of the flexible sheets 26 a to 26 d. In addition, as shown inFIGS. 1 to 3B, the board portion 12 includes the resist film 20, lands28, wiring conductors 30 (30 c), a ground conductor 37, and via-holeconductors b1 to b3 and b21 to b26. In FIGS. 1 to 3B, with regard to thelands 28, the wiring conductors 30, and the via-hole conductors b1 tob3, the reference signs are provided to representative ones in order toprevent the drawings from being complicated.

As shown in FIG. 2, each land 28 is a conductor layer provided on thefront surface of the board portion sheet 27 a. As shown in FIG. 1, thechip elements 50 and the integrated circuit are mounted on the lands 28preferably via soldering, for example.

The resist film 20 is arranged so as to cover the front surface of theboard portion sheet 27 a, and is an insulating film provided to protectthe board portion sheet 27 a. It should be noted that the resist film 20preferably is not provided on the lands 28. The resist film 20 isproduced, for example, by applying a resin.

As shown in FIG. 2, each wiring conductor 30 c preferably is a conductorlayer provided on the front surface of the board portion sheet 27 c. Asshown in FIGS. 2 and 3A, the via-hole conductors b1 to b3 are arrangedso as to extend through the board portion sheets 27 a to 27 c,respectively, in the z-axis direction. By the board portion sheets 27 ato 27 d being laminated, the lands 28, the wiring conductors 30 c, andthe via-hole conductors b1 to b3 are connected to each other to form acircuit.

As shown in FIG. 2, the via-hole conductors b21 to b23 are arranged soas to extend through the board portion sheet 27 d in the z-axisdirection. In addition, as shown in FIG. 2, the via-hole conductors b24to b26 are arranged so as to extend through the board portion sheet 27 cin the z-axis direction. The via-hole conductors b24 to b26 areconnected to the via-hole conductors b21 to b23, respectively.

The ground conductor 37 is a conductor layer provided within the boardportion 12. Specifically, the ground conductor 37 preferably is arectangular film-shaped electrode arranged so as to cover the frontsurface of the board portion sheet 27 d. It should be noted that asshown in FIG. 2, the ground conductor 37 does not cover the entiresurface of the board portion sheet 27 d and is not provided near theouter periphery of the board portion sheet 27 d. In addition, the groundconductor 37 is grounded to be maintained at a ground potential. Theground conductor 37 is connected via the via-hole conductor b3 to thecircuit that includes the lands 28, the wiring conductors 30 c, and thevia-hole conductors b1 to b3.

When seen in a planar view from the z-axis direction, as shown in FIG.2, the resist film 20 and the ground conductor 37 have the same shapesand overlap each other so as to coincide with each other. When seen in aplanar view from the z-axis direction, the lands 28 are provided in aregion in which the resist film 20 and the ground conductor 37 areprovided. Thus, as shown in FIG. 1, the chip elements 50 and theintegrated circuit 52 are mounted in the region in which the resist film20 and the ground conductor 37 are provided.

Next, the board portion 14 will be described. As shown in FIG. 2, theboard portion 14 preferably includes laminated board portion sheets 29 ato 29 d of the flexible sheets 26 a to 26 d. In addition, as shown inFIGS. 1 to 3B, the board portion 14 includes the resist film 24, lands35, wiring conductors 36 (36 c), a ground conductor 40, and via-holeconductors b11, b12, and b31 to b36. In FIGS. 1 to 3B, with regard tothe lands 35, the wiring conductors 36 c, and the via-hole conductorsb11 and b12, the reference signs are provided to representative ones inorder to prevent the drawings from being complicated.

As shown in FIG. 2, each land 35 is a conductor layer provided on thefront surface of the board portion sheet 29 a. As shown in FIG. 1, theconnector 54 is mounted on the lands 35 preferably via soldering, forexample.

The resist film 24 is arranged so as to cover the front surface of theboard portion sheet 29 a, and is an insulating film provided to protectthe board portion sheet 29 a. It should be noted that the resist film 24is preferably not provided on the lands 35. The resist film 24 isproduced, for example, by applying a resin.

As shown in FIG. 2, each wiring conductor 36 c is a line-shapedconductor layer provided on the front surface of the board portion sheet29 c. As shown in FIGS. 2 and 3A, the via-hole conductors b11 and b12are arranged so as to extend through the board portion sheets 29 a and29 b, respectively, in the z-axis direction. By the board portion sheets29 a to 29 d being laminated, the lands 35, the wiring conductors 36 c,and the via-hole conductors b11 and b12 are connected to each other toform a circuit.

As shown in FIG. 2, the via-hole conductors b31 to b33 are arranged soas to extend through the board portion sheet 29 b in the z-axisdirection. In addition, as shown in FIG. 2, the via-hole conductors b34to b36 are arranged so as to extend through the board portion sheet 29 cin the z-axis direction. The via-hole conductors b34 to b36 areconnected to the via-hole conductors b31 to b33, respectively.

The ground conductor 40 is a conductor layer provided within the boardportion 12. Specifically, the ground conductor 40 preferably is arectangular film-shaped electrode arranged so as to cover the frontsurface of the board portion sheet 29 d. It should be noted that asshown in FIG. 2, the ground conductor 40 does not cover the entiresurface of the board portion sheet 29 d and is not provided near theouter periphery of the board portion sheet 29 d. In addition, the groundconductor 40 is grounded to be maintained at the ground potential.

When seen in a planar view from the z-axis direction, as shown in FIG.2, the resist film 24 and the ground conductor 40 have the same shapesand overlap each other so as to coincide with each other. When seen in aplanar view from the z-axis direction, the lands 35 are provided in aregion in which the resist film 24 and the ground conductor 40 areprovided. Thus, as shown in FIG. 1, the connector 54 is mounted in theregion in which the resist film 24 and the ground conductor 40 areprovided.

Next, the line portion 16 will be described. As shown in FIG. 2, theline portion 16 includes laminated line portion sheets 31 a to 31 d ofthe flexible sheets 26 a to 26 d. In addition, as shown in FIGS. 1 and2, the line portion 16 includes ground lines (32 b and 32 d), 33 (33 band 33 d), and 34 (34 b and 34 d) and signal lines 42 c, 43 c, and 44 c.

The signal lines 42 c, 43 c, and 44 c are provided within the lineportion 16 and extend between the board portions 12 and 14.Specifically, as shown in FIG. 2, the signal lines 42 c, 43 c, and 44 cpreferably are line-shaped conductor layers provided on the frontsurface of the line portion sheet 31 c. In the signal lines 42 c, 43 c,and 44 c, high-frequency signals (e.g., 800 MHz to 900 MHz) aretransmitted. As shown in FIG. 2, the signal lines 42 c, 43 c, and 44 cconnect the wiring conductors 30 c to the wiring conductors 36 c.

The ground lines 32 b, 33 b, and 34 b are provided within the lineportion 16 and provided on the positive direction side in the z-axisdirection with respect to the signal lines 42 c, 43 c, and 44 c.Specifically, as shown in FIG. 2, the ground lines 32 b, 33 b, and 34 bare provided on the front surface of the line portion sheet 31 b andconnect the via-hole conductors b21, b22, and b23 to the via-holeconductors b31, b32, and b33, respectively. Thus, the ground lines 32 b,33 b, and 34 b are connected to the ground conductor 37 via the via-holeconductors b21 to b26, respectively. In addition, the ground lines 32 b,33 b, and 34 b are connected to the ground conductor 40 via the via-holeconductors b31 to b36, respectively.

Further, as shown in FIG. 4, the ground lines 32 b, 33 b, and 34 b haveline widths (i.e., widths in the y-axis direction) equal to those of thesignal lines 42 c, 43 c, and 44 c (it should be noted that in FIG. 4,only the ground line 33 b and the signal line 43 c are shown). Theground lines 32 b, 33 b, and 34 b overlap the signal lines 42 c, 43 c,and 44 c, respectively, so as to coincide therewith, when seen in aplanar view from the z-axis direction.

The ground lines 32 d, 33 d, and 34 d are provided within the lineportion 16 and provided on the negative direction side in the z-axisdirection with respect to the signal lines 42 c, 43 c, and 44 c.Specifically, as shown in FIG. 2, the ground lines 32 d, 33 d, and 34 dare provided on the front surface of the line portion sheet 31 d andconnect the ground conductor 37 to the ground conductor 40.

Further, as shown in FIG. 4, the ground lines 32 d, 33 d, and 34 d haveline widths larger than those of the signal lines 42 c, 43 c, and 44 c(it should be noted that in FIG. 4, only the ground line 33 d and thesignal line 43 c are shown). Thus, when seen in a planar view from thez-axis direction, the signal lines 42 c, 43 c, and 44 c overlap theground lines 32 d, 33 d, and 34 d, respectively, without protruding fromthe ground lines 32 d, 33 d, and 34 d.

As described above, the ground lines 32 b, 33 b, and 34 b, the signallines 42 c, 43 c, and 44 c, and the ground lines 32 d, 33 d, and 34 doverlap each other. Thus, the ground line 32 d, the signal line 42 c,and the ground line 32 d constitute a strip line. Similarly, the groundline 33 b, the signal line 43 c, and the ground line 33 d constitute astrip line. The ground line 34 b, the signal line 44 c, and the groundline 34 d constitute a strip line. Thus, impedance matching is providedbetween the circuit within the board portion 12 and the circuit withinthe board portion 14.

Further, as shown in FIGS. 1 and 2, in the circuit board 10, the groundconductors 37 and 40 are provided in the board portions 12 and 14. Theground conductors 37 and 40 serve to reinforce the board portions 12 and14. Thus, the board portions 12 and 14 have structures that are lesslikely to deform as compared to the line portion 16. The chip elements50, the integrated circuit 52, and the connector 54 are mounted on theboard portions 12 and 14 that are less likely to deform.

When being mounted in an electronic device, the circuit board 10 can bebent in a U shape. Hereinafter, the circuit board 10 that is bent in a Ushape will be described with reference to the drawings. FIG. 5A is across-sectional structure diagram of the circuit board 10 that is bentin a U shape. FIG. 5B is an enlarged diagram at B in FIG. 5A.

As shown in FIG. 5A, the circuit board 10 is mounted in an electronicdevice in a state of being bent in a U shape. In other words, since theline portion 16 is bent in a U shape once, the board portion 14 extendson the lower side of the board portion 12. At that time, as shown inFIG. 5B, the ground lines 32 b, 33 b, and 34 b are provided on the outerperipheral side of the line portion 16 with respect to the ground lines32 d, 33 d, and 34 d.

Hereinafter, a method of manufacturing the circuit board 10 will bedescribed with reference to the drawings. In the following, the casewhere one circuit board 10 is manufactured will be described as anexample. In reality, a plurality of circuit boards 10 are simultaneouslymanufactured by laminating and cutting large flexible sheets.

First, flexible sheets 26 including copper foil provided on the entiretyof the front surface thereof are prepared. Next, a laser beam is appliedfrom the back surface side to locations in the flexible sheets 26 a to26 c where the via-hole conductors b1 to b3, b11, b12, b21 to b26, andb31 to b36 (see FIGS. 2 and 3A) are to be formed, to form via holes.

Next, the lands 28 and 35 shown in FIG. 3B are formed on the frontsurface of the flexible sheet 26 a by a photolithographic process.Specifically, resists having the same shapes as those of the lands 28and 35 shown in FIG. 3B are printed on the copper foil of the flexiblesheet 26 a. Then, etching treatment is performed on the copper foil toremove the portion of the copper foil that is not covered with theresists. Then, the resists are removed. By so doing, the lands 28 and 35are formed on the front surface of the flexible sheet 26 a as shown inFIG. 3B. Further, a resin is applied to the front surface of theflexible sheet 26 a to form the resist films 20 and 24 shown in FIGS. 1and 2.

Next, the ground lines 32 b, 33 b, and 34 b shown in FIG. 2 are formedon the front surface of the flexible sheet 26 b by a photolithographicprocess. Further, the wiring conductors 30 c and 36 c and the signallines 42 c, 43 c, and 44 c shown in FIG. 2 are formed on the frontsurface of the flexible sheet 26 c by a photolithographic process.Further, the ground lines 32 d, 33 d, and 34 d and the ground conductors37 and 40 shown in FIG. 2 are formed on the front surface of theflexible sheet 26 d by a photolithographic process. Thesephotolithographic processes are the same as the photolithographicprocess performed when forming the lands 28 and 35, and thus thedescription thereof is omitted.

Next, the via holes formed in the flexible sheets 26 a to 26 c arefilled with a conductive paste containing copper as a principalcomponent, to form the via-hole conductors b1 to b3, b11, b12, b21 tob16, and b31 to b36 shown in FIGS. 2 and 3A.

Next, the flexible sheets 26 a to 26 d are stacked in order. Then,forces are applied to the flexible sheets 26 a to 26 d from the upperand lower directions in the z-axis direction, to pressure-bond theflexible sheets 26 a to 26 d. By so doing, the circuit board 10 shown inFIG. 1 is obtained.

According to the circuit board 10 described above, unwanted emission canbe prevented as described below. Specifically, as shown in FIG. 4, theground lines 32 b, 33 b, and 34 b are provided on the positive directionside in the z-axis direction with respect to the signal lines 42 c, 43c, and 44 c, and the ground lines 32 d, 33 d, and 34 d are provided onthe negative direction side in the z-axis direction with respect to thesignal lines 42 c, 43 c, and 44 c. The ground lines 32 b, 33 b, and 34b, the signal lines 42 c, 43 c, and 44 c, and the ground lines 32 d, 33d, and 34 d overlap each other when seen in a planar view from thez-axis direction. Thus, noise emitted from the signal lines 42 c, 43 c,and 44 c is absorbed by the ground lines 32 b, 32 d, 33 b, 33 d, 34 b,and 34 d and is hardly emitted to the outside of the circuit board 10.

In the circuit board 10, the line widths of the ground lines 32 b, 33 b,and 34 b are the same as the line widths of the signal lines 42 c, 43 c,and 44 c. Thus, it is thought that unwanted emission cannot sufficientlybe suppressed as compared to the strip line 200 b shown in FIG. 8B.However, the circuit board 10 can sufficiently prevent unwanted emissiondue to the following reason. Superficially, in the circuit board 10, asshown in FIG. 4, the corners of the ground lines 32 b, 33 b, and 34 bare close to the corners of the signal lines 42 c, 43 c, and 44 c. As aresult, due to the edge effect, most of electric force lines E generatedfrom the corners of the signal lines 42 c, 43 c, and 44 c extend towardthe corners of the ground lines 32 b, 33 b, and 34 b. Thus, the electricforce lines E generated from the signal lines 42 c, 43 c, and 44 c passthrough the ground lines 32 b, 33 b, and 34 b on the positive directionside or the negative direction side in the y-axis direction and hardlyextend toward the outside of the circuit board 10. Due to this, thecircuit board 10 can prevent unwanted emission.

Moreover, in the circuit board 10, as described below, even though theground lines 32 b, 33 b, and 34 b, the signal lines 42 c, 43 c, and 44c, and the ground lines 32 d, 33 d, and 34 d constitute the strip lines,the line portion 16 can easily be bent in a U shape. Specifically, inthe circuit board 10, the ground lines 32 b, 33 b, and 34 b have thesame line widths as those of the signal lines 42 c, 43 c, and 44 c.Meanwhile, in the existing strip line 200 b shown in FIG. 8B, the groundconductor 208 has a line width larger than that of the signal line 204.Thus, when the signal line 43 c and the signal line 204 have equal linewidths, the ground lines 32 b, 33 b, and 34 b have line widths smallerthan that of the ground conductor 208. As a result, in the circuit board10, it is possible to easily bend the line portion 16 in a U shape ascompared to the strip line 200 b. As described above, the circuit board10 can prevent unwanted emission and can easily be bent in a U shape.

Further, the circuit board 10 can easily be bent in a U shape due to thefollowing reason as well. Specifically, as shown in FIG. 5B, in the bentline portion 16, the stretch amounts of the ground lines 32 b, 33 b, and34 b, the signal lines 42 c, 43 c, and 44 c, and the ground lines 32 d,33 d, and 34 d increase from the inner peripheral side toward the outerperipheral side. Thus, in the circuit board 10, of the ground lines 32b, 32 d, 33 b, 33 d, 34 b, and 34 d, the ground lines 32 b, 33 b, and 34b that are easily stretchable due to their small line widths areprovided on the outer peripheral side of the ground lines 32 d, 33 d,and 34 d. This makes it possible to easily bend the circuit board 10 ina U shape.

Further, the circuit board 10 can prevent entry of noise, generated atthe chip elements 50 and the integrated circuit 52, into the connector54, and entry of noise, generated at the connector 54, into the chipelements 50 and the integrated circuit 52. Specifically, as shown inFIGS. 5A and 5B, the mounting surfaces of the board portions 12 and 14do not face each other. Thus, the ground conductors 37 and 40 arepresent between the mounting surface of the board portion 12 and themounting surface of the board portion 14. The ground conductors 37 and40 are grounded and hence serve as shields. Therefore, noise generatedat the chip elements 50 and the integrated circuit 52 is absorbed by theground conductors 37 and 40, and entry of the noise into the connector54 is prevented. Similarly, noise generated at the connector 54 isabsorbed by the ground conductors 37 and 40, and entry of the noise intothe chip elements 50 and the integrated circuit 52 is prevented.

Hereinafter, modified examples of the line portion 16 will be describedwith reference to the drawings. FIG. 6 is a cross-sectional structurediagram of a line portion 16 a according to a first modified example.FIG. 7 is a cross-sectional structure diagram of a line portion 16 baccording to a second modified example.

As shown in FIG. 6, the ground lines 32 b, 33 b, and 34 b may have linewidths smaller than those of the signal lines 42 c, 43 c, and 44 c (inFIG. 6, only the ground line 33 b and the signal line 43 c are shown).In this case, the widths d1, in the y-axis direction (in the line widthdirection), of the portions of the signal lines 42 c, 43 c, and 44 cthat protrude from the ground lines 32 b, 33 b, and 34 b when seen in aplanar view from the z-axis direction, are smaller than the intervals d2in the z-axis direction between the ground lines 32 b, 33 b, and 34 band the signal lines 42 c, 43 c, and 44 c. This is because when thewidth d1 is larger than the interval d2, the edge effect becomes weakand hence the electric force lines generated from the signal lines 42 c,43 c, and 44 c do not extend toward the ground lines 32 b, 33 b, and 34b.

In the line portion 16 a shown in FIG. 6, when seen in a planar viewfrom the z-axis direction, the ground lines 32 b, 33 b, and 34 bdesirably overlap the signal lines 42 c, 43 c, and 44 c withoutprotruding from the signal lines 42 c, 43 c, and 44 c. By so doing, thecorners of the signal lines 42 c, 43 c, and 44 c can be made close tothe corners of the ground lines 32 b, 33 b, and 34 b and hence it ispossible to greatly exert the edge effect.

However, the above description does not exclude protrusion of the groundlines 32 b, 33 b, and 34 b from the signal lines 42 c, 43 c, and 44 cwhen seen in a planar view from the z-axis direction as in the lineportion 16 b shown in FIG. 7.

The widths of the ground lines 32 d, 33 d, and 34 d shown in FIG. 6 haveto be such widths that at least the electric force lines extending fromthe signal lines 42 c, 43 c, and 44 c toward the ground lines 32 d, 33d, and 34 d perpendicularly enter the surfaces of the ground lines 32 d,33 d, and 34 d that face the signal lines 42 c, 43 c, and 44 c. In otherwords, the widths d3, in the y-axis direction (the line widthdirection), of the portions of the ground lines 32 d, 33 d, and 34 dthat protrude from the signal lines 42 c, 43 c, and 44 c when seen in aplanar view from the z-axis direction have to be as large as about halfthe intervals d4 in the z-axis direction between the signal lines 42 c,43 c, and 44 c and the ground lines 32 d, 33 d, and 34 d. For example,when d4 is about 100 μm, d3 is preferably about 50 μm. By so doing, theelectric force lines extending from the signal lines 42 c, 43 c, and 44c toward the ground lines 32 d, 33 d, and 34 d perpendicularly orsubstantially perpendicular enter the surfaces of the ground lines 32 d,33 d, and 34 d that face the signal lines 42 c, 43 c, and 44 c, and donot extend around to the surfaces of the ground lines 32 d, 33 d, and 34d that are parallel or substantially parallel to the z-axis directionand to the surfaces of the ground lines 32 d, 33 d, and 34 d that areopposite to the surfaces of the ground lines 32 d, 33 d, and 34 d thatface the signal lines 42 c, 43 c, and 44 c. Thus, it is easy to designthe impedance of the signal transmission line that is a componentbetween a microstrip line and a strip line. Further, variation of theimpedance of the signal transmission line due to processing can bereduced, and a stable signal transmission line can be achieved.

The circuit board 10 has been described. However, the structures of theline portions 16, 16 a, and 16 b are applicable to things other than thecircuit board 10. Specifically, the structures of the line portions 16,16 a, and 16 b may be used for a signal transmission line (cable)connecting between circuit boards.

Preferred embodiments of the present invention are useful for signaltransmission lines and circuit boards, and in particular, are excellentin being able to easily be bent in a U shape and to reduce unwantedemission.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing from the scopeand spirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

What is claimed is:
 1. A signal transmission line comprising: a lineportion including insulating sheets made of a flexible material; asignal line extending within the line portion; a first ground lineprovided within the line portion on one side in a lamination directionwith respect to the signal line and having a line width equal to orsmaller than a line width of the signal line; and a second ground lineprovided within the line portion on another side in the laminationdirection with respect to the signal line; wherein the signal lineoverlaps the first ground line and the second ground line when seen in aplanar view from the lamination direction.
 2. The signal transmissionline according to claim 1, wherein the second ground line has a linewidth larger than the line width of the signal line, and the signal lineoverlaps the second ground line without protruding from the secondground line, when seen in a planar view from the lamination direction.3. The signal transmission line according to claim 1, wherein the firstground line overlaps the signal line without protruding from the signalline, when seen in a planar view from the lamination direction.
 4. Thesignal transmission line according to claim 1, wherein a width, in aline width direction, of a portion of the signal line that protrudesfrom the first ground line when seen in a planar view from thelamination direction is smaller than an interval in the laminationdirection between the signal line and the first ground line.
 5. Thesignal transmission line according to claim 2, wherein the line portionis bent, and the first ground line is provided on an outer peripheralside of the bent line portion with respect to the second ground line. 6.A circuit board comprising: a main body that includes a first boardportion and second board portion on which electronic components aremounted, and a line portion connecting the first board portion to thesecond board portion and that includes a plurality of laminatedinsulating sheets made of a flexible material; a signal line providedwithin the line portion and extending between the first board portionand the second board portion; a first ground line provided within theline portion on one side in a lamination direction with respect to thesignal line and having a line width equal to or smaller than a linewidth of the signal line; and a second ground line provided within theline portion on another side in the lamination direction with respect tothe signal line; wherein the signal line overlaps the first ground lineand the second ground line when seen in a planar view from thelamination direction; and the first board portion and the second boardportion are less deformable than the line portion.
 7. The circuit boardaccording to claim 6, further comprising: a first ground conductorprovided within the first board portion and connected to the secondground line; and a second ground conductor provided within the secondboard portion and connected to the second ground line; wherein the firstboard portion and the second board portion are provided with the firstground conductor and the second ground conductor, respectively, and areless likely to deform than the line portion.